1. Field of the Invention
The present invention relates to high density memory devices based on phase change based memory materials, including chalcogenide based materials and on other programmable resistive materials, and to methods for manufacturing such devices.
2. Description of Related Art
Programmable resistive materials, including phase change based materials, have been used in nonvolatile random access memory cells. Phase change materials, such as chalcogenides, can be caused to change phase between an amorphous state and a crystalline state by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher resistivity than the generally crystalline state, which can be readily sensed to indicate data.
Phase change materials are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in the active region of the cell. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has detectable characteristics such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched into either different solid phases or mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states.
The change from the amorphous to the crystalline state is generally a lower current operation, requiring a current that is sufficient to raise the phase change material to a level between a phase transition temperature and a melting temperature. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, allowing at least a portion of the phase change structure to stabilize in the amorphous state. It is desirable to minimize the magnitude of the reset current used to cause transition of phase change material from a crystalline state to an amorphous state. The magnitude of the needed reset current can be reduced by reducing the volume of the active region in the phase change material element in the cell. Techniques used to reduce the volume of the active region include reducing the contact area between electrodes and the phase change material, so that higher current densities are achieved in the active volume, with small absolute current values through the phase change material element.
One direction of development has been toward forming small pores in an integrated circuit structure, and using small quantities of programmable resistive material to fill the small pores. Patents illustrating development toward small pores include: Ovshinsky, “Multibit Single Cell Memory Element Having Tapered Contact,” U.S. Pat. No. 5,687,112, issued 11 Nov. 1997; Zahorik et al., “Method of Making Chalogenide [sic] Memory Device,” U.S. Pat. No. 5,789,277, issued 4 Aug. 1998; Doan et al., “Controllable Ovonic Phase-Change Semiconductor Memory Device and Methods of Fabricating the Same,” U.S. Pat. No. 6,150,253, issued 21 Nov. 2000.
Another memory cell structure under development, referred to sometimes as a “mushroom” cell because of the shape of the active region on the bottom electrode in a typical structure, is based on the formation of a small electrode in contact with a larger portion of phase change material, and then a usually larger electrode in contact with an opposite surface of the phase change material. Current flow from the small contact to the larger contact is used for reading, setting and resetting the memory cell. The small electrode concentrates the current density at the contact point, so that an active region in the phase change material is confined to a small volume near the contact point. See, for example, Ahn et al., “Highly reliable 50 nm contact cell technology for 256 Mb PRAM,” VLSI Technology 2005 Digest of Technical Papers, pages 98-99, 14 Jun. 2005; Denison, International publication No. WO2004/055916 A2, “Phase Change Memory and Method Therefore,” Publication Date: 1 Jul. 2004; and Song et al., United States Patent Application Publication No. US 2005/0263829 A1, “Semiconductor Devices Having Phase Change Memory Cells, Electronic Systems Employing the Same and Methods of Fabricating the Same,” published 1 Dec. 2005.
Another problem with manufacturing very small dimension structures is alignment. When the structures are made using separate lithographic steps, the sizes of the structures, or of at least one of them, must be large enough to allow for alignment tolerances in the lithographic process. These requirements can restrict the flexibility in the design of the memory cells, and cause variation in the performance of the cells.
A self-aligned, nonvolatile memory structure based upon phase change material is described in U.S. Pat. No. 6,579,760 entitled Self-Aligned Programmable Phase Change Memory, invented by Hsiang-Lan Lung, issued Jun. 17, 2003. The memory structure can be made within a very small area on an integrated circuit. For example, the area required for each memory cell in an array is about 4F2, where F is equal to the minimum line width for the manufacturing process. Thus, for processes having a minimum line width of 0.1 microns, the memory cell area is about 0.04 microns squared.
Memory cells, including a stack of materials forming diode access devices and a layer of phase change material, are defined at intersections of bit lines and word lines, and have dimensions that are defined by the widths of the bit lines and word lines in a self-aligned process. However, the dimensions of the word lines and bit lines are still quite large, as compared for example to the size of a pore in a pore-type memory cell. Thus it is desirable to provide a high-density array technology, using self-aligned technology, and which provides for formation of very small pores
It is desirable therefore to provide a reliable method for manufacturing a memory cell structure with self-aligning and self-converging control over the critical dimensions of the pore cell, which will work with high density integrated circuit memory devices.